1. Field of the Invention
The present invention relates to a method and apparatus for lighting an electro-luminescence (EL) element, and more particularly, to a method and apparatus for lighting an EL element, having a small size with high brightness.
2. Description of the Related Art
An (EL) element has the advantages in a thin type, a light weight, ad a flexible shape and has used for back-lighting a liquid crystal device. AC voltage of about 100 and about 800 Hz is required to light the EL device. In order to obtain such AC voltage, a DC-AC inverter is generally used for converting DC voltage into AC voltage. As such an inverter, a self-excited type of inverter has used which includes a blocking oscillating circuit composed of a transformer, transistors, resistors and capacitors, as proposed in Japanese Examined Patent Application Publication, JP-B-Sho-62-15032. In addition, there is proposed an inverter in Japanese Examined Patent Application Publication, JP-B-sho-62-11314, in which the EL element has an area as small as 10 cm.sup.2 and is driven with a single dry cell.
In the self-excited type inverter including a blocking oscillating circuit, the circuit is simple but a ratio of the primary winding and the secondary winding of a transformer must become greater as a supply DC voltage becomes smaller, resulting in requiring the large transformer in size. In addition, when the area of the EL element is reduced, the drive frequency of the EL element is increased if the inductance of the secondary winding is the same. However, since the drive frequency cannot be increased to a large extent with respect to the life and efficiency, the drive frequency is kept to be about 800 Hz by increasing the inductance of the secondary winding of the transformer. In this case where the EL element of 10 cm.sup.2 is driven with 800 Hz, the inductance of, for example, 10 H or more is necessary. Therefore, the transformer cannot be reduced in size.
Next, the inverter proposed in the JP-B-Sho-62-11314 will be described below with reference to FIG. 1.
The inverter mainly includes a charging circuit 1010, a discharging circuit 1020 and a control circuit 1030. The charging circuit 1010 includes a boosting circuit composed of an inductor L1, a bipolar transistor Tr1 and an oscillating circuit 1011 for turning on/off the transistor TR1, and a diode D1 for rectifying the boosted current, and the discharging circuit 1020 is composed of a transistor Tr2.
Next, the operation of the inverter will be described below. When the control circuit 1030 supplies a charge control signal to the oscillating circuit 1011 as shown in FIG. 2A, the oscillating circuit 1011 oscillates in response to the charge control signal as shown in FIG. 2B and supplies the oscillation signal to the base of the transistor Tr1. When the oscillation signal is in the high level, the transistor Tr1 is turned on to flow current through the inductor L1. At this time, electromagnetic energy of (1/2).multidot.L.multidot.I.sup.2 is stored in the inductor L1, where L is inductance of the inductor L1 and I is a peak value of the current flowing through the inductor L1. Subsequently, when the oscillated signal goes to the low level, the transistor Tr1 is turned off such that the electromagnetic energy stored in the inductor L1 is transferred to the capacitive EL element through the diode D1 and stored therein. In this manner, by repeating the switching operation of the oscillation signal between the high level and low level, the EL element is charged step by step as shown in FIG. 2D. In this state, the charge control signal is changed to the low level, so that the oscillating circuit 1011 stops the oscillation. While the oscillation is stopped in the oscillating circuit 1011, the control circuit 1030 supplies a discharge control signal to the discharging circuit 1020 as shown in FIG. 2C. The transistor Tr2 of the discharging circuit 1020 is turned on to make the EL element to discharge the stored charge. A single period of operation in the inverter is completed as described above. The EL element lights twice, i.e., upon charging and discharging in the single period.
In the EL element lighting device using a low voltage power supply such as a dry battery as in JP-B-Sho-62-11314, since the oscillation frequency is required to be higher by 2 digits than the drive frequency of the EL element, the inductance of the inductor L1 must be about a few hundreds of .mu.H. This value is smaller by 5 digits than that of the transformer in the above self-excited type of inverter including the blocking oscillating circuit. However, the EL element lighting device needs the control circuit in addition to the charging circuit and the discharging circuit. In addition, the control circuit needs to generate the two different, complicatedly controlled signals for controlling the charging circuit and the discharging circuit. As a result of this, the lighting device becomes complicated so that it cannot be down sized, resulting in the high cost of the lighting device.
In addition, in a case that a low voltage power supply such as a dry battery is used, the switching element for driving the inductor L1 is limited to a bipolar transistor having a low operation voltage. Since the bipolar transistor generally has a large turn off time of, for example, more than 1 .mu.s, a part of current to be flowed into the EL element flows through the bipolar transistor during the transition of the bipolar transistor from the on state to the off state. As a result of this, the charging energy for the EL element is reduced such that the luminous efficiency is degraded, which makes it difficult to achieve high bright illumination of the EL element.